The present invention relates generally to thin film photovoltaic devices and in particular the invention provides a structure and a method of forming the structure for thin film cells to achieve light trapping in these cells.
The use of light trapping is well know in monocrystalline silicon cells where light trapping features in the surface of the cell have dimensions which are much less than the thickness of silicon substrate of the device and significantly greater than the wavelength of light in air.
In solar cells, light scattering is used to trap light in the active region of the cell. The more light that is trapped in the cell, the higher the efficiency that can be obtained. Therefore, light trapping is an important issue when trying to improve efficiency in solar cells and is particularly important in thin film cell design.
However it has been widely thought that in thin film devices where the active silicon layers are thin films formed over a substrate such as glass, light trapping would not be possible or at least demonstrate reduced effectiveness. This is because the film thicknesses are of the same order of magnitude or thinner than the dimension of light trapping features in known monocrystalline devices. As film thicknesses in thin film devices are reduced, they tend toward conformal coatings having predominantly parallel surfaces over the etched surface of the glass substrate and conventional thinking would be that such an arrangement does not achieve significant advantages from light trapping. Also as film thicknesses are reduced to the order of a wavelength (in air) or thinner conventional thinking is that the mechanisms providing light trapping in prior art devices would cease to be effective.
This is born out in prior art thin film amorphous silicon solar cell devices where no deliberate attempt at texturing was made. Present day amorphous silicon devices typically comprise a glass superstrate over which is layed a Transparent Conductive Oxide (TCO) contact layer and a thin amorphous silicon film (1 xcexcm) active layer including a p-n junction and a rear metallic layer acting as a reflector and back contact. When such structures were first devised it was noticed that in some circumstances (when the surface of the TCO was cloudy) cell performance was greater than expected but the literature offered no explanation as to the reason for such unexpected performance.
It has now become apparent to the present inventors that light trapping is possible in thin film devices and the prior art amorphous silicon devices were in fact demonstrating a characteristic that the present inventors have now identified and adapted to crystalline silicon thin film cells. Central to the invention is the realisation that the wavelength of light of a given frequency is different in silicon and air.
The present inventors have now identified the circumstances under which light trapping can be achieved in thin films and in particular the inventors have devised methods for manufacture of thin film crystalline silicon solar cell structures which exhibit light trapping characteristics.
According to a first aspect, the present invention provides a method of forming a light trapping structure in a thin film silicon solar cell formed on a glass substrate or superstrate, the method including the steps of:
a) etching a surface of the glass substrate or superstrate to provide a textured surface; and
b) forming a silicon film on the textured surface and forming a photovoltaic device structure in the silicon film, the silicon film being less than 10 xcexcm thick.
According to a second aspect, the present invention provides a thin film photovoltaic device incorporating a light trapping structure, wherein the photovoltaic device is formed on a textured surface of a glass substrate or superstrate, the photovoltaic device comprising a thin silicon film into which is formed at least one pn photovoltaic junction, the silicon film being less than 10 xcexcm thick.
The invention is applicable to both crystalline and amorphous silicon solar cells. In the case of amorphous silicon devices, a Transparent Conducting Oxide (TCO) layer will be formed immediately below the silicon film.
In embodiments of the invention, the textured surface of the glass preferably includes surface features having dimensions in the range of from 0.05-0.2 times the thickness of the silicon film. Preferably also, a thin conformal barrier layer is formed between the textured glass surface and the silicon film to prevent migration of contaminants from the glass to the silicon.
The barrier layer may also act as an anti-reflection layer and will in that case, be arranged to have a thickness equal to a quarter wavelength xc2x120% which in the case of silicon nitride is 70 nmxc2x120%
The silicon film is typically less than 5 xcexcm thick and preferably has a thickness of 2 xcexcm or less. The silicon film is typically at least 0.5 xcexcm or greater and preferably greater than 1 xcexcm. Typically, the scale of textured surface features is in the range of 0.01-10 xcexcm. The useful lower limit of the feature size is in the order of a wavelength of light in crystalline silicon and typically the useful lower limit is 0.05 xcexcm. The texturing may also include large scale features which have dimensions greater than the thickness of the silicon film.
In some embodiments of the invention, the surface of the substrate is textured by an etching step, which is performed to form features in the substrate surface having a range of dimensions including dimensions which are greater than the thickness of the silicon film structure. In preferred embodiments these features are in the range of 1-5 xcexcm on average and typically in the order of 2 xcexcm.
Where such an etching step is employed it is performed in a manner to additionally provide small scale surface features which have dimensions significantly less than the thickness of the silicon film structure. Typically, the small scale surface features will vary in the range of 0.01-1 xcexcm.
In one embodiment of the invention, the etching step is performed by applying an etching composition which includes HF or a derivative thereof to the glass surface to be etched.
The glass substrate, after etching using HF may be optionally subsequently heated to a temperature at which the glass softens and angular edges of the surface features are rounded. An application of a layer of spin-on glass, can also be used instead of thermal rounding of the etched glass surface.
In typical embodiments of the invention the back surface of the silicon film structure (ie, remote from the glass) has a reflective material formed over it. Typically, the reflective material will be a metallisation structure used to contact the active regions of the cell. The metallisation structure will in some embodiments, be separated from most of the silicon back surface by an insulating layer.